Electronic device having electric wires and method of producing same

ABSTRACT

An electronic device such as a chip coil including an electric wire firmly connected to electrodes in a highly reliable fashion is constructed to be mounted on a printed circuit board or substrate in a stable and reliable manner. At both ends of a core of the chip coil, there are provided electrodes having a multilayer structure including a high-conductivity layer made of Ag, Ag--Pd, or a similar material; a solder barrier layer made of Ni; and an easy-soldering layer made of Sn or solder. End portions of the electric wire are embedded in the easy-soldering layer so that the resultant electrode structure has a substantially flat surface. A thermo-compression process is performed so that the end portions of the electric wire are connected to the solder barrier layer via solid welding and to the easy-soldering layer via brazing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic device having electricwires such as a wire-wound chip coil and to a method of producing such acoil device. The present invention also relates to a method of producingan electronic device having electric wires in an inductance component.More particularly, the present invention relates to a method ofproducing an electronic device having electric wires including animproved process of connecting end portions of an insulated electricwire wound around a core to electrodes located on the core.

2. Description of the Related Art

FIG. 5 illustrates a conventional chip coil made up of an electric wire5 wound around a core 1 made of a magnetic material, wherein ends 5a ofthe wire 5 are connected, via thermo-compression bonding such as a wirebonding process, to respective electrodes 2 located on the core 1. Theelectrodes 2 are made of a material such as Ag or Ag--Pd. When the ends5a of the wire are connected to the respective electrodes 2, theresultant connecting portions of the wire 5 have raised portions bulgingfrom the surface of the electrodes 2. The bulging shape of theconnecting portions causes the chip coil to become unstable when it ismounted on a printed circuit board. That is, the chip coil is mountedunevenly in a slanted orientation or topples over and is separated fromthe printed circuit board in the worst case. Another problem with thechip coil of this type is that the ends 5a of the wire 5 are exposeddirectly to air and thus, the ends of the wire are oxidized. This makesit difficult to solder the ends of the wire during the process ofmounting the chip coil.

One possible technique of improving the stability of the mountedposition is to form recesses 3 as seen in FIG. 6 in both end portions ofthe core 1 so that the ends 5a of the wire can be placed inside therecesses 3. However, the shape of the core 1 becomes complicated anddifficult processes are required to produce such a complicated structureincluding the recesses 3.

The insulated electric wire 5 generally consists of an electric wiremade of metal such as copper whose outer surface is coated with aninsulating material such as polyesterimide. The insulated electric wireis connected to the electrodes via, for example, pulse heating. Theconnection process via pulse heating is described below with referenceto FIG. 11.

In FIG. 11, cross sections of the core 51 of the coil device and theelectrode 52 located on the upper surface of the core 51 are shown. Anend portion of the insulated electric wire 53 is disposed on theelectrode 52.

A pressing tip 56 heated at about 500° C. is moved down so that theinsulated electric wire 53 is pressed against the electrode 52. Theelectric wire 53a is flattened by the pressure and the electric wire 53ais connected to the electrode 52 via thermo-compression bonding.

In this connection technique, if the electrode 52 is made of metalhaving a high melting point such as Ag, Cu, or Ni, the insulatingcoating 53b melts at a temperature lower than the melting point of theelectrode 52, and the electric wire 53a and the electrode 52 aredirectly connected to each other. Another feature of this technique isthat the electric wire 53a is flattened by the pressure.

However, although the electric wire 53a on the electrode 52 isflattened, there is still a processing step required on the surface ofthe electrode 52 and the electric wire 53a. When the coil device ismounted on a printed circuit board such that the surface of theelectrode 52 and attached electric wire 53a comes into contact with theprinted circuit board, the above-described step can cause the coildevice to become unstable or cause the soldered connection to becomeunreliable.

In many cases, the surface of the electrodes 52 of the coil device isplated with metal having a low melting point such as Sn or solder sothat a low-melting-point electrode layer is formed on the electrode 52thereby ensuring that the electrode can be easily soldered. For example,as shown in FIG. 12, the electrode 52 is produced by coating asilver-filled paste on the surface of the core 51 and baking it so as toform a base layer 52a, then plating the surface of the base layer 52awith Ni thereby forming a Ni-plated layer 52b for protecting the baselayer 52a from being eroded by solder, and finally forming an electrodelayer 52c of low-melting-point metal which allows the electrode 52 to beeasily soldered.

In the case of the coil device having the above structure, whenconnection is performed with the pressing tip 54 heated at about 500°C., the electrode layer 52c is heated by the pressing tip 54 to atemperature higher than the melting point of the low-melting-pointmetal. In the connecting process, the electrically conductive wire 53ais pressed by a pressure high enough to compress the electricallyconductive wire 53a. As a result, the insulating coating 53b and theelectrode layer 52c made of the low-melting-point metal at the top layerare both melted into liquid states, and the electrically conductive wire53a is compressed into a flattened shape.

As a result, in the pressing process using the pressing tip 54, thelow-melting-point metal in the liquid state is pushed aside by theinsulating coating 53b in the liquid state toward the sides of theelectrically conductive wire 53a. In the above process, after thelow-melting-point metal is pushed aside by the melted insulating coating53b, if the melted insulating coating 53b ticks to the pressing tip 54and is removed when the pressing tip 54 is moved up to its originalposition, the Ni-plated layer 52b a sometimes exposed in an area A at aside of the conductive wire 53a. If the Ni-plated layer 52b is partiallyexposed, when the coil device is mounted on a printed circuit board viasoldering, a connection failure can occur because the Ni-plated layer52b has poor solder wettability.

Even in the case where the Ni-plated layer 52b does not become exposedin the area A in FIG. 12 after the low-melting-point metal is pushedaside by the melted insulating coating 53b, if the melted insulatingcoating 53b sticks to the pressing tip 54 and is removed when thepressing tip 54 is moved up to its original position, a crater 52d isproduced in an area on the surface of the electrode 52 where theinsulating coating 53b of the electrically conductive wire 53a waspresent. If such a crater is produced, it becomes difficult to make agood connection in the soldering process.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide an electronic device including a wire which ismounted in a stable fashion, firmly connected to an electrode and isvery resistant to oxidization and a method of producing such anelectronic device.

In addition, preferred embodiments of the present invention provide amethod of producing a coil device, capable of connecting an insulatedelectric wire to an electrode for connection to an outer circuit in sucha manner that the insulated electric wire is embedded in the electrodethereby ensuring that the coil device can be connected to the outercircuit in a highly reliable fashion without causing a soldering failureor any problem which occurs in the conventional techniques.

According to a preferred embodiment of the present invention, anelectronic device includes an electric wire firmly connected to anelectrode located on an insulating base, the electrode including atleast a solder barrier layer made of a material with a high meltingpoint and an easy-soldering layer made of a material with a low meltingpoint; and the electric wire is embedded via a thermo-compressionprocess in the easy-soldering layer in such a manner that the resultantstructure has a substantially flat surface.

In the electronic device according to preferred embodiments of thepresent invention, the flattened electric wire is embedded in theeasy-soldering layer so that the resultant structure of the electrodehas a flat surface including no raised portions extending from theelectrode. As a result of this structure, the electronic device can bemounted in a stable fashion on a circuit board without causing theelectronic device to be slanted or to topple over.

In the electronic device according to preferred embodiments of thepresent invention, the surfaces of the easy-soldering layer and electricwire are preferably covered with another easy-soldering layer so thatthe surfaces of the electrodes become flatter and so that the surfacesof the electrodes are protected from being oxidized.

According to another aspect of preferred embodiments of the presentinvention, there is provided a method of producing an electronic device,including forming an electrode on an insulating base, the electrodeincluding at least a solder barrier layer made of a material with a highmelting point and an easy-soldering layer made of a material with a lowmelting point disposed on the surface of the solder barrier layer; andpressing the electric wire against the electrode while heating theelectric wire and the electrode so that the electric wire and the solderbarrier layer are connected together via solid welding and so that theelectric wire and the easy-soldering layer are connected together viabrazing.

In this production method according to preferred embodiments of theinvention, the easy-soldering layer is melted via heating and compressedvia pressing so that the electric wire sinks into the easy-solderinglayer. As a result, the electrode has a structure having a substantiallyflat surface in which the electric wire and the solder barrier layer areconnected to each other via solid welding and the electric wire and theeasy-soldering layer are connected to each other via brazing. Thistechnique makes it possible to connect the electric wire to theelectrode in a highly reliable manner. In the case where the electricwire is covered with an insulating coating such as polyesterimide, theinsulating coating is melted/vaporized via heating in the aboveproduction process. Therefore, no additional process for removing thecoating is necessary.

In the connection process described above, it is preferable that theelectric wire and the electrode be heated so that the temperaturethereof increases in a very short period of time to a value higher thanthe melting point of the easy-soldering layer and lower than the meltingpoint of the solder barrier layer, and then cooled so that thetemperature decreases in a very short time to a value lower than themelting point of the easy-soldering layer. Although the time periods ofheating, sustaining the elevated temperature, and cooling vary slightlydepending on the diameter and material of the electric wire, each timeperiod is preferably set to a value shorter than about a few seconds andmore preferably shorter than about 1 second.

According to still another aspect of preferred embodiments of thepresent invention, there is provided a method of producing a coildevice, the coil device including a core made of an insulating material,an insulated electric wire wound around the core, and electrodesdisposed on the outer surface of the core, the end portions of theinsulated electric wire being connected to the respective electrodes,the method including a low-pressure pressing step in which the insulatedelectric wire is pressed against the electrodes by a relatively lowpressure at a temperature high enough to melt the insulating coating ofthe insulated electric wire; and a high-pressure pressing step performedfollowing the low-pressure pressing step, in which the electric wire ispressed against the electrode by a relatively high pressure so that theelectric wire becomes flat and so that the flattened electric wire isembedded in the electrode.

The temperature in the high-pressure pressing step may be set to a valuehigher than the melting point of the top portion of the electrode andlower than the freezing point thereof.

The high-pressure pressing step may be performed at a temperature eitherlower or higher than the temperature at which the low-pressure pressingstep is performed.

Preferably, the electrode of the coil device includes a base layerpreferably formed via coating and baking an electrically conductivepaste such as a silver-filled paste, a solder barrier layer disposed onthe surface of the base layer using a metal material such as Ni havinghigh resistance to erosion by solder, and an easy-soldering layerdisposed on the outer surface of the solder barrier layer using a metalmaterial such as Sn or solder which is easy to solder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a first preferred embodiment of a chipcoil according to the present invention;

FIG. 1B is a plan view of the preferred embodiment of the chip coilshown in FIG. 1A;

FIG. 1C is a cross-sectional view taken along line C--C in FIG. 1B;

FIG. 2A is a cross-sectional view illustrating an example of aproduction method according to a preferred embodiment of the presentinvention in a state immediately before a thermo-compression process;

FIG. 2B is a cross-sectional view of the process shown in FIG. 2A in astate during the thermo-compression process;

FIG. 3 is a cross-sectional view of the process shown in FIG. 2Aillustrating a modification of the first preferred embodiment;

FIG. 4A illustrates a method of producing a chip coil according to asecond preferred embodiment of the present invention in a stateimmediately before a thermo-compression process;

FIG. 4B illustrates the method shown in FIG. 4A in a state immediatelyafter the thermo-compression process;

FIG. 5 is a perspective view illustrating an example of a conventionalchip coil;

FIG. 6 is a perspective view illustrating another example of aconventional chip coil;

FIG. 7 is a cross-sectional view illustrating the processing steps ofconnecting an insulated electric wire to a first electrode according toa third preferred embodiment of the invention;

FIG. 8 is a perspective view illustrating a coil device produced by amethod according to the third preferred embodiment of the invention;

FIG. 9 is a graph illustrating the profile of the pressure andtemperature applied in the preferred embodiment shown in FIG. 7;

FIG. 10 is a cross-sectional view illustrating a high-pressure pressingprocess according to another preferred embodiment of the invention;

FIG. 11 is a cross-sectional view illustrating a process for connectingan insulated electric wire to an electrode according to a conventionaltechnique; and

FIG. 12 is a cross-sectional view illustrating a problem which occurswhen an insulated electric wire is connected to an electrode made of ametal material having a low melting point according to a conventionaltechnique.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The electronic device and the production method according to preferredembodiments of the present invention are described below with referenceto preferred embodiments in conjunction with the accompanying drawings.

FIG. 1 illustrates a first preferred embodiment of a chip coil accordingto the present invention. In this chip coil, an electric wire 15 iswound around the main portion 11 of a ceramic core 10, and end portions16 of the wire 15 are firmly connected, preferably viathermo-compression bonding, to electrodes 13 located on raised portions12 at both ends of the core 10.

As shown in FIG. 1C, each electrode 13 is preferably formed as follows.First, a high-conductivity material such as Ag or Ag--Pd is coated onthe core 10 and baking is performed thereby forming a high-conductivitylayer 13a. The surface of the high-conductivity layer 13a iselectroplated with Ni so as to form a solder barrier layer 13b.Furthermore, the surface of the solder barrier layer 13b iselectroplated with Sn or solder so as to form an easy-soldering layer13c.

The wire 15 is preferably made of a conductor of Cu with a diameter ofabout 20 to about 60 μm and is preferably covered with an insulatingmaterial such as polyesterimide. The end portions 16 of the wire 15 areconnected, preferably via thermo-compression, to the respectiveelectrodes 13 in an embedded arrangement as seen in FIG. 1C.

The process of connecting the wire 15 to the electrodes 13 is describedbelow.

As shown in FIG. 2A, the end portions 16 of the wire are placed on therespective electrodes 13 and pressed from above by a heater 20 so thatthe end portions 16 are heated by the heater 20. The pressing andheating are preferably performed at the same time for both end portions16 of the wire. In a short period of time, for example, less than about1 sec, the temperature of the heater 20 is increased to a value higherthan the melting point of the easy-soldering layer 13c (the meltingpoint of Sn is 231° C. and that of solder is 183° C.) and lower than themelting point of the solder barrier layer 13b (the melting point of Niis 1455° C.), and more preferably, to a temperature higher than 500° C.The temperature is maintained at this value for about 1 sec or for ashorter time period. The temperature of the heater 20 is decreasedquickly in a short period of time, preferably less than about 1 sec to avalue lower than the melting point of the easy-soldering layer 13c. Thenthe heater 20 is moved away from end portions 16 of the wire. Forexample, a pulse heating heater may be used as the heater 20. With thistype of heater, heating can be performed by supplying a pulse currentwhile precisely controlling the heating and pressing conditions. Themelting points of Cu and Ag are 1083° C. and 960.5° C., respectively.

Instead of pulse heating, ultrasonic wave radiation may be used toperform the above heating process.

In the connecting process described above, the insulating coating of theend portions 16 of the wire is melted/vaporized by heat. The surface ofthe end portions 16 of the wire melts and becomes soft. Theeasy-soldering layer 13c also melts. The end portions 16 of the wire areflattened by the pressure and sink into the easy-soldering layer 13c(see to FIG. 1C). In this state, the end portions 16 of the wire and thesolder barrier layer 13b are connected to each other at a contact plane17a via solid-phase welding, and the end portions 16 of the wire and theeasy-soldering layer 13c are connected to each other at a contact plane17b via brazing.

After completion of the connecting process, the electrode has astructure in which the end portion 16 of the electric wire and theeasy-soldering layer 13c become substantially flush and thus, there isno raised portion of the wire extending up from the electrode 13. Thisstructure makes it possible to mount the chip coil on a printed circuitboard in a stable fashion without the chip coil being slanted or beingprone to topple over or to be removed from the printed circuit board.Furthermore, because the end portions 16 of the electric wire areconnected to the electrodes 13 via solid welding and brazing, theconnections are highly reliable. Furthermore, the insulating coating ofthe end portions 16 of the electric wire are removed during the heatingprocess and no additional process or step for removing the coating isnecessary. The heating is performed for a very short time so that theinsulating coating of the electric wire 15 in the winding part is notdamaged.

In some cases, as shown in FIG. 1C, the melted easy-soldering layer 13cflows toward the end portions 16 of the electric wire and thus, the endportions 16 of the electric wire are covered by the easy-soldering layer13c. This results in a further improvement in the flatness of thesurface of the electrode 13 and also prevents the end portions 16 of theelectric wire from being oxidized. In particular, after completion ofthe above connecting process, if another easy-soldering layer 13d of Snor solder is formed via plating or other suitable process on the surfaceof the electrode 13 as shown in FIG. 3, then the flatness is furtherimproved and the end portions 16 of the electric wire are prevented frombeing oxidized in a more reliable fashion. In the case where theeasy-soldering layer 13d is formed via plating, the size of the endportions 16 of the electric wire are reduced by the plating bath. As aresult, the solderability is further improved by the synergistic effectsof the presence of the easy-soldering layer 13d and the reduction of theend portions 16 of the wire.

FIG. 4 illustrates a second preferred embodiment of a chip coilaccording to the present invention. The second preferred embodimentdiffers from the first preferred embodiment described above in that theelectrode 13' disposed on the core 10 has a two-layer structureincluding of a solder barrier layer 13b made of Ni and an easy-solderinglayer 13c made of Sn or solder. The end portions 16 of the electric wireare connected preferably via a thermo-compression bonding processsimilar to that used in the first preferred embodiment so that the endportions 16 of the electric wire and the solder barrier layer 13b areconnected to each other at a connecting plane 17a via solid welding, andthe end portions 16 of the electric wire and the easy-soldering layer13c are connected to each other at a connecting plane 17b via brazingFurthermore, as in the first preferred embodiment, the end portions 16of the electric wire are connected to the electrodes in an embeddedarrangement so that the resultant structure has a flat surface. Aftercompletion of the connecting process described above, the surface of theresultant electrode structure may be covered with another easy-solderinglayer made of Sn or solder.

FIGS. 7A-7D are cross-sectional views illustrating the process ofproducing a coil device according to a third preferred embodiment of thepresent invention. FIG. 8 is a perspective view of the coil device madeby the production method according to the third preferred embodiment ofthe present invention.

First, the structure of the chip type coil device shown in FIG. 8 isdescribed below.

The chip type coil device 31 includes a core 32 made of a dielectricmaterial, magnetic material, or an insulating material such asinsulating ceramic or plastic. The core 32 includes a winding part 32aaround which an insulated electric wire 33 is wound and electrode parts32b located at both ends of the winding part 32a wherein the electrodeparts 32b have a greater thickness than the winding part 32a. First andsecond electrodes 34 and 35 are disposed on the upper surface of theelectrode portions 32b and 32c, respectively, so that electricalconnections to external components can be achieved via the first andsecond electrodes 34 and 35.

The method of producing a coil device, such as the chip type coil device31 described above, according to the present preferred embodiment of theinvention includes a novel process of connecting the insulating wire 33to the first and second electrodes 34 and 35. The other processes may beaccomplished by known techniques. That is, the process of forming thewinding part by winding the insulated electric wire 33 around the outersurface of the core 32 and the process of forming the electrodes 34 and35 on the upper surface of the electrode parts 32b and 32c of the core32 may be performed using known techniques.

The process of connecting the end portions of the insulated electricwire 33 to the first and second electrodes 34 and 35, which is a novelfeature of preferred embodiments of the present invention, is describedin detail below. The process for connecting only one end of theinsulated electric wire 33 to the first electrode 34 is described forthe purpose of simplicity although both end portions are preferablyconnected according to the novel process described below.

First, as shown in FIG. 7a, the insulated electric wire 33 is placed onthe first electrode 34 located on the upper surface of the electrodepart 32b of the core 32 of the coil device. The first electrode 34preferably has a multilayer structure including of a base layer 34a, aNi-plated layer 34b, and a Sn-plated layer 34c. The base layer 34a isformed by coating, for example, a silver-filled conductive paste andthen baking. Alternatively, the base layer 34a may also be formed viaevaporation, plating, sputtering, or other suitable techniques.

The Ni-plated layer 34b disposed on the base layer 34a functions as asolder barrier layer for protecting the base layer 34a from being erodedby solder. Instead of Ni, Cu or Fe may also be used for the samepurpose.

The Sn-plated layer 34c disposed on the outer surface of the Ni-platedlayer 34b functions as an easy-soldering layer which makes it easy toperform soldering when the coil device is mounted. Instead of Sn, solderor other materials which can be easily soldered may also be used to formthe plated layer functioning as the easy-soldering layer.

In the present preferred embodiment, the insulated electric wire 33includes an electrically conductive wire 33a made of Cu covered with aninsulating coating 33b. The insulating coating 33b may be formed usingpolyesterimide or similar material. When polyesterimide is used to formthe insulating coating 33b, it can be melted at a temperature equal toor higher than 330° C.

When the insulated electric wire 33 is connected to the first electrode34, the insulated electric wire 33 is first placed on the firstelectrode 34 and a pressing tip 36 is moved down to the wire 33, whereinthe pressing tip 36 is heated at a temperature high enough to melt theinsulating coating 33b on the electric wire. In this specific preferredembodiment, the tip 36 is preferably heated at about 500° C. or a highertemperature. In the above process, the pressing tip 36 is moved down sothat the insulated electric wire 33 is pressed against the firstelectrode 34 by a relatively low pressure. The optimum pressure in thispressing process depends on the diameter and material of the insulatedelectric wire 33. In an example in which the insulated electric wire ismade of Cu and has a diameter of about 40 μm, the pressure is preferablyset to a value within the range of about 30 to about 50 gF. Thus, in thepreferred embodiments of the present invention, the low-pressurepressing process is preferably performed in the manner described above.

In the low-pressure pressing process, the insulating coating 33b meltsbecause the pressing tip 36 is heated at a temperature high enough tomelt the insulating coating. As a result, as shown in FIG. 7b, themelted insulating coating 33b on the first electrode 34 moves to bothsides of the electrically conductive wire 33a, and thus the lower sideof the electrically conductive wire 33a comes into direct contact withthe Sn-plated layer 34c which is the top layer of the electrode 34.

In the above low-pressure pressing process, as shown in FIG. 7b,substantially no deformation occurs in the electrically conductive wire33a because the pressure applied by the pressing tip 36 is low enough.That is, the low-pressure pressing process is performed at a lowpressure at which the electrically conductive wire 33a does not becomeflat.

Furthermore, in the low-pressure pressing process, because the Sn-platedlayer 34c is not in direct contact with the pressing tip 36 and becausethe pressure applied by the pressing tip 36 is low, and because thepressing process is performed in a rather short period of time as shownin FIG. 9, the Sn-plated layer 34c is not melted although the insulatingcoating 33b is melted. In other words, the pressure and the pressingtime are selected so that the insulating coating 33b is melted but theSn-plated layer 34c is not melted.

Following the low-pressure pressing process, a high-pressure pressingprocess is performed at a relatively high pressure. In thishigh-pressure pressing process, the electrically conductive wire 33a ispressed against the first electrode 34 by a high enough pressure so thatthe electrically conductive wire 33a is flattened and embedded into thefirst electrode 34. The high-pressure pressing process is described infurther detail below with reference to FIGS. 7c and 7d.

In the high-pressure pressing process, as shown in FIG. 7c, the pressureapplied to the pressing tip 36 in a downward direction is increased sothat the electrically conductive wire 33a is made flattened. That is, inthe high-pressure pressing process, the pressure applied by the pressingtip 36 to the electrically conductive wire 33a is selected so that theelectrically conductive wire 33a becomes flat and so that the flattenedwire 33a is embedded into the first electrode 34.

Thus, in the above high-pressure pressing process, the electric wire 33ais made flat and embedded into the first electrode 34 as shown in FIG.7d.

Preferably, the temperature in the high-pressure pressing process is setto a value lower than the melting point of the top layer of the firstand second electrodes, and more specifically, lower than the meltingpoint of the Sn-plated layer 34c, and higher than the freezing point ofthe Sn-plated layer 34c. In this specific preferred embodiment, thetemperature is set to about 230° C. If the temperature in thehigh-pressure pressing process is selected within the above range, itbecomes possible to make the Sn-plated layer 34c of the first electrode34 soft enough so that the electrically conductive wire 33a is embeddedinto the first electrode 34.

If the temperature in the high-pressure pressing process is set to avalue higher than the melting point of the Sn-plated layer 34c, then theSn-plated layer 34c is melted during the high-pressure pressing processand melted tin is pushed aside by the insulating coating 33b. As aresult, there is a possibility that some portion of the Ni-plated layer34b is exposed or a crater is produced after the insulating coating 33bis removed.

The optimum pressure in the high-pressure pressing process depends onthe diameter and the material of the electrically conductive wire 33a Inan example in which the electrically conductive wire is made of Cu andhas a diameter of about 40 μm, the pressure is preferably set to about300 gF.

After completion of the high-pressure pressing process, the pressing tip36 is moved upward. As a result, the insulating coating 33b sticking tothe lower surface 36a of the pressing tip 36 is removed from the firstelectrode. As shown in FIG. 7d, after the insulating coating 33b isremoved, the electrode 34 has a flat surface having no craters.

FIG. 9 illustrates the profile of the temperature and the pressure inthe low-pressure pressing process and also in the high-pressure pressingprocess. As can be seen from FIG. 9, it is preferable to perform thehigh-pressure pressing process immediately after the low-pressurepressing process so that the electrode 34 has a flat surface after theend portion of the insulated electric wire 33 is connected to theelectrode 34.

According to the production method of the present preferred embodiment,as described above, the electric wire 33a becomes flat and the flattenedwire 33a is embedded into the first electrode 34. As a result, the firstelectrode 34 has a flat surface after the insulated electric wire 33 isconnected to the first electrode 34. Therefore, the coil device can bemounted on a printed circuit board or the like in such a manner that theelectrodes are connected via solder or the like to the printed circuitboard in a highly reliable manner.

Furthermore, because the Sn-plated layer 34c is exposed at the areaoutside the electrically conductive wire 33a, excellent solderabilitycan be achieved.

Although in the above described preferred embodiment, the high-pressurepressing process is performed at a temperature lower than thetemperature used in the low-pressure pressing process, the temperaturein the high-pressure pressing process may be higher than that in thelow-pressure pressing process. For example, in the case where theinsulating coating 33b is made of a urethane resin, the urethane resinmelts at about 280° C. however the Sn-plated layer does not become softat such a temperature. In this case, the urethane resin insulatingcoating is melted in the low-pressure pressing process, and theSn-plated layer is made soft in the high-pressure pressing process byusing a higher temperature in the high-pressure pressing process thanthe temperature in the low-pressure pressing process thereby ensuringthat the electrically conductive wire is embedded into the Sn-platedlayer of the first electrode, as in the preferred embodiment describedabove.

Although in the third preferred embodiment described above the first andsecond electrodes are incorporated into the multilayer structureincluding the base layer 34a made of the silver-filled conductive paste,the Ni-plated layer 34b, and the Sn-plated layer 34c, the first andsecond electrodes may also be formed to have a single layer structure.

That is, the first and second electrodes may be formed of a single metalmaterial and the insulated electric wire may be connected to the firstand second electrodes by performing the low-pressure pressing processand the high-pressure pressing process. Also in this case, as in thecase of the third preferred embodiment described above, in thelow-pressure pressing process performed first, the electricallyconductive wire does not become flat and the insulating coating ismelted and pushed aside. Then in the high-pressure pressing process, arelatively high pressure is applied by the pressing tip 36 to theelectrically conductive wire 33a so that the wire 33a is embedded intothe electrode 24, as shown in FIG. 10. To ensure that the wire 33a canbe embedded, it is preferable that the temperature in the high-pressurepressing process is higher than the temperature at which the firstelectrode 24 made of the single material becomes soft, that is, thefreezing temperature, and should be lower than the melting point of thefirst electrode 24.

The insulating coating 33b melts and moves to the sides of the wire 33aif the low-pressure pressing process is performed first. When thepressing tip 36 is moved up after completion of the high-pressurepressing process performed after the low-pressure pressing process, theinsulating coating 33b is removed because the insulating coating 33bsticks to the lower surface 36a of the pressing tip 36. Therefore, asshown on the right side of FIG. 10, after the completion of thehigh-pressure pressing process, the first electrode 24 has a structurein which the wire 33a is connected to the first electrode 24 in such amanner that the surface of the connecting part becomes flat.

Although in the above preferred embodiment the coil device has astructure in which a pair of electrodes are provided at both ends of thecore around which the insulated electric wire 33 is wound and the bothends of the insulated electric wire 33 are connected to the respectiveelectrodes, the present invention is also applicable to a coil devicehaving a structure in which electrodes are disposed only at one side ofa core 32 and the ends of an insulated electric wire are connected tothe electrodes. Furthermore, there is no limitation on the number ofelectrodes to which the ends of the insulated electric wire areconnected.

Although in the above-described preferred embodiment the temperature inthe low-pressure pressing process is preferably set to a value at whichthe insulating coating 33b melts but the Sn-plated layer 34c or thelow-melting-point metal layer hardly melts at all, the temperature inthe low-pressure pressing process may also be set to a value at whichthe low-melting-point metal layer 34c melts. In the case where thetemperature in the low-pressure pressing process is set to a value atwhich the low-melting-point metal layer 34c melts, the low-melting-pointmetal layer 34c may be cooled, in the following process, to atemperature higher than its freezing point so that the low-melting-pointmetal layer 34c is in a soft state thereby ensuring that the wire 33a isembedded into the electrode in the high-pressure pressing process, as inthe above-described preferred embodiment. The cooling may be performedeither during or prior to the high-pressure pressing process.

The electric wire 33a may also be made of metal other than copper. Forexample, Ni or Ag may be used for this purpose. Furthermore, theinsulating material of the insulating coating 33b is not limited topolyesterimide or urethane resins, but other proper synthetic resins mayalso be used.

Although the present invention has been described above with referenceto specific preferred embodiments, the invention is not limited to thedetails described therein. The electronic device and the productionmethod therefor may be modified in various ways without departing fromthe spirit and the scope of the invention.

For example, the present invention can be applied not only to a chipcoil which is designed to be mounted in a horizontal position as is thecase in the first to third preferred embodiments, but also to a chipcoil which is mounted in a vertical position (in which the winding axisof the coil extends in a direction substantially perpendicular to theplane on which the coil is mounted). Furthermore, instead of themagnetic core 10, a dielectric core or an insulated core made ofceramics or resin may also be used to form the insulating basestructure. Still furthermore, the present invention can be applied notonly to the chip coil but also to a wire-wound inductor. Furthermore,the present invention may be applied to a wide variety of electronicdevices combined with other types of electric functional devices such asa capacitor.

As can be understood from the above description, the present inventionhas various advantages. That is, in the present invention, the electrodeon the insulating base includes at least a solder barrier layer made ofa material having a high melting point and an easy-soldering layer madeof a material having a low melting point, and end portions of theelectric wire are embedded via thermo-compression process into theeasy-soldering layer in such a manner that the resultant electrodestructure has a substantially flat surface having no raised portionsextending up from the electrode. This structure makes it possible tomount the coil device on a printed circuit board in a stable manner. Inparticular, if the surface of the end portions of the wire are coveredwith another easy-soldering layer after they are connected to theelectrode, then the end portions of the electric wire are prevented fromoxidation and thus, solderability is further improved.

Furthermore, in the connecting process according to preferredembodiments of the present invention, the end portions of the electricwire are pressed against the electrode while being heated so that theelectric wire and the solder barrier layer are connected to each othervia solid welding and the electric wire and the easy-soldering layer areconnected to each other via brazing. This allows the electric wire to beconnected to the electrode in a highly reliable manner. Therefore, theelectronic device including such a connection has high reliability andhigh resistance to mechanical shocks and vibrations. Furthermore, theinsulating coating of the electric wire is melted/evaporated by heatduring the connecting process, and no additional process for removingthe coating is necessary.

In the method of producing a coil device according to preferredembodiments of the present invention, the low-pressure pressing processis preferably performed first so that the end portions of the insulatedelectric wire are pressed by a relative low pressure against theelectrode at a temperature high enough to melt the insulating coatingthereby melting the insulating coating and moving the melted coatingtoward the sides of the electric wire. In the following high-pressurepressing process, a higher pressure is applied to the electric wire sothat the end portions of the electric wire become flat and are embeddedinto the electrode. Thus, the resultant connections have a structureincluding a flat surface, in which the electric wire is connected to theelectrodes in a highly reliable fashion. As a result, highly reliableelectric connections are achieved and the coil device can be mounted ona printed circuit board or the like via soldering in a highly reliablefashion.

Still furthermore, in preferred embodiments of the present invention,the high-pressure pressing process is preferably performed at atemperature lower than the melting point of the top layer of theelectrodes and higher than the freezing point thereof so that thesurface of the electrodes becomes soft in the high-pressure pressingprocess thereby ensuring that the end portions of the electric wire areembedded into the electrodes.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of producing an electronic component,comprising the steps of:forming at least one electrode on a base, saidelectrode including at least a solder barrier layer made of a materialhaving a high melting point and an easy-soldering layer made of amaterial having a low melting point disposed on the surface of saidsolder barrier layer; and pressing an electric wire against the at leastone electrode while heating the electric wire and the electrode so thatthe electric wire and the solder barrier layer are connected togethervia solid welding and so that the electric wire and the easy-solderinglayer are connected together via brazing.
 2. A method of producing anelectronic component, according to claim 1, wherein in said step ofpressing the electrically conductive wire against the electrode whileheating, the electrically conductive wire and the electrode are heatedso that the temperature thereof increases to a value higher than themelting point of the easy-soldering layer and lower than the meltingpoint of the solder barrier layer, and then the electrically conductivewire and the electrode are cooled so that the temperature decreases to avalue lower than the melting point of the easy-soldering layer.
 3. Amethod of producing a coil device, according to claim 2, wherein saidhigh-pressure pressing step is performed at a temperature lower than atemperature at which said low-pressure pressing step is performed.
 4. Amethod of producing a coil device, according to claim 2, wherein saidhigh-pressure pressing step is performed at a temperature higher than atemperature at which said low-pressure pressing step is performed.